Andy,
Yes, I think $35USD is doable, recognizing you will be living lean. I tried to plan around $40 USD/day. Frankly, I went a little over that, but I also didn't live as lean as I might have. For me, one "rule" I had for myself was I had to sleep in a private room. I was willing to share a bathroom as necessary, but with a laptop and expensive camera was not willing to sleep in a shared room. That obviously meant a higher expense on hostels or hotels. I also probably ate at restaurants (if casual or fast food) more than I needed to. I could have opted for buying from markets and eating food I prepared myself or didn't need preparation for (such as eating more veggies from markets, which I probably should have done anyway).
Not that these are large expenses, but consider bus or other travel expenses, add border crossing fees (I think Nicaragua was $7/person, Costa Rica something like $5. Not major amounts but a bite on the day you cross from your $35). Look for restaurants where the locals eat as opposed to the tourists, which means some venturing out to find the local neighborhoods...you'll save money that way too. If you decide to do any tours, obviously be mindful as that will quickly eat into a daily budget. Think about things you may have to buy such as soap, toothpaste, etc. Think about laundry expenses. I did not find many, if any, laundromats for self-service so you'll probably have an expense to pay someone to wash your clothes. Again, not a major expense at around the equivalent of $1-2 per pound of clothes but an expense.
Finally, especially if you speak reasonably good Spanish I don't think you'll have too much trouble finding locals to talk with and while there are quality tours and "organized" experiences that are "authentic", I agree sometimes they are a bit too scripted and "non-authentic".
A specific suggestion if you make it to Nicaragua...
Quetzaltrekkers. They have a "full moon" volcano trek overnight that looked fascinating. I sadly did not make it due to the timing of when I was in Nicaragua. For me to be in Nicaragua over a full moon to do the trek up the mountain I was going to have to be in Nicaragua for about another two weeks with a combination of running out of things to do during those two weeks and a bit of homesickness as it was the end of my travels, so I passed. However, I was otherwise ready to do it. If you do it, you'll have to email me and tell me if it's worth it. It would be a bite into your budget, however, at $30/person.
Finally, another website to check out if you haven't already is
Hostelworld. This is the one I used to find most of my places to stay. Usually found decent budget accommodations.
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